A battery pack generally includes a battery, terminals for connecting a battery charger or a lead, a protection circuit for detecting various conditions of the battery such as conditions of potential hazard to the battery, and switches for controlling connection and disconnection of the battery. Typically, a battery pack utilizes a single transistor, generally a field effect transistor (FET), as a switch for controlling connection of the battery. Unfortunately, a single transistor typically conducts some current through an associated body-drain diode so that the battery may not be fully disconnected when disconnection is intended. When the battery is not fully disconnected, unexpected and inappropriate voltages may be applied at the terminals of the battery pack circuit.
Furthermore, the operating potential of a protect circuit is typically only a fraction of the battery voltage. In contrast, a much larger potential difference is applied to a switch for connecting and disconnecting the battery from the pack. This voltage mismatch between the protect circuit and the switch creates a problem. The control signal generated by the protect circuit may be inappropriate for controlling the switch. This problem is exacerbated by the possibility that an illegal charger, a charger having an unsuitable voltage with respect to the battery voltage, may be applied to the battery. For example, a 12 volt charger, or possibly an illegal charger having a voltage greater than 12 volts, may be applied to a battery pack which includes a protect circuit operating at 4 volts. A signal generated by the protect circuit must be suitable for deactivating and activating a FET switch. In this example, a node at which the control signal is applied may be pulled down by the charger, possibly to a -12 volt level with respect to the ground potential of the protect chip. Nevertheless, the control signal must produce a suitable control signal to connect and disconnect the battery by activating and deactivating a FET or a pair of serial FETS.
In addition, battery pack circuits often require an active (logic 1) signal to turn off switches and disconnect the battery. However, this is wasteful of power since it requires an expenditure of power when a load is not connected.
Furthermore, previous battery pack switching approaches require two pins in a control integrated circuit for controlling a switch. Generally, it is advantageous to reduce the number of pins in an integrated circuit since battery pack size is very important for portable applications.
What is required is a switch for controlling battery disconnecting and battery conduction in a battery pack, which is reliably activated and deactivated for connecting and disconnecting the battery despite large voltage mismatches between the switch and the circuit generating a control signal. What is also required is a switch which substantially eliminates current conduction when the battery is disconnected. It is advantageous for such a switch to utilize an active signal to connect, rather than disconnect, the battery. Also, it is important that the control circuit fails safe, that is, in the event of a mechanical disconnect or complete loss of power due to a battery shorting, the FETs would be turned off and left off. This is important for safety.